Normal Knee Range of Motion

The knee has been described as having 6 degrees of freedom—internal-external rotation, compression-distraction, medial-lateral translation, abduction-adduction, anterior-posterior translation, and flexion-extension. Normal extension is 0 degrees, but some individuals may have slight hyperextension of up to 5 degrees. Normal knee flexion is 140 degrees in men and 143 degrees in women. Knee flexion up to 165 degrees is seen in certain groups.¹ The functional range of motion for most activities of daily living is 10 to 120 degrees. Loss of knee flexion or extension can lead to significant disability and inability to perform simple every day activities. Studies looking at gait patterns have shown that 67 degrees of knee flexion is required in the swing phase of walking, 83 degrees to ascend stairs, 90 degrees to descend stairs, 93 degrees to rise from a standard chair, and 110 degrees to cycle upright.²

The knee joint is a complex articulation among the femur, tibia, and the patella. The knee consists of two menisci, two collateral ligaments, and two cruciate ligaments. There are multiple specific areas within the knee that have the potential to develop fibrosis leading to stiffness. The treatment of a stiff knee is based on these specific areas that are most often affected. It is important to understand the anatomy and pathoanatomy associated with each of these areas.

Potential Locations of Entrapment

Suprapatellar Pouch.

The suprapatellar pouch, also referred to as the suprapatellar bursa, is the superior continuation of the knee joint. This area is posterior to the quadriceps tendon and anterior to the femur. It continues on either side of the patella as the medial and lateral gutters. This area is covered by a synovial membrane. The suprapatellar pouch should extend 3 to 4 cm above the patella. It is the most common area of arthrofibrosis. With extensive scarring of the suprapatellar pouch, knee flexion is typically lost (Fig. 4-1). Terminal extension is usually not affected as much.

FIGURE 4-1 Arthroscopic view of suprapatellar pouch fibrosis.

Anterior Interval.

This potential site of entrapment includes the fat pad, patellar tendon, and retropatellar recess. The anterior interval is bordered anteriorly by the fat pad and patellar tendon, posteriorly by the intracondylar notch and anterior border of the tibial plateau, superiorly by the patella, and inferiorly by the tibial tubercle. The retropatellar recess is a space between the insertion of the patellar tendon and anterior border of the tibia. Extracapsular structures in this area include the medial and lateral patellotibial ligaments. These structures can become contracted, leading to progressive arthrofibrosis of the knee. When fibrosis and scarring occur in the anterior interval, the patella may be drawn inferiorly, creating patella baja. However, patella baja, or patella infera, may not always occur with arthrofibrosis unless a dense band of scar forms between the inferior pole of the patella and anterior aspect of the tibial plateau (Fig. 4-2). This pseudopatellar tendon can severely hinder knee motion and lead to patella tendon resorption. Loss of flexion and extension may be present in those patients with anterior interval involvement (Fig. 4-3).

FIGURE 4-2A,B Arthroscopic views of infrapatellar entrapment. P, patella; *, scar tissue forming a pseudotendon between patella and tibia; T, trochlea.

FIGURE 4-3 Limited patellar motion when pseudotendon is present (arrows).

Potential Causes of Knee Stiffness